1. Field of the Invention
This invention relates to a high stability interferometer for measuring small changes in the refractive index of gaseous or liquid samples and to a measuring method using the interferometer.
2. Prior Art Statement
Refractive index measurement is used widely by production industries, such as for determining the concentration of solutions and monitoring the progress of chemical reactions. With small changes in the refractive index of samples often being indicative of major changes, there is a need for systems which are capable of providing continuous measurement of changes on refractive index in the order of 10 .sup.-7 or less.
Conventionally, the refractive index is measured by determining the critical angle at which total internal reflection occurs, at an interface between the sample solution and a transparent substance (usually glass) whose refractive index is accurately known. Although this method can provide a good level of measurement precision, its implementation as a sampling or batch oriented technique makes it unsuitable for the on-line measurement of refractive index in production processes.